The present disclosure is related to methods of laser processing of materials. More particularly, the present disclosure is related to methods of singulation and/or cleaving of wafers, substrates, and plates.
In current manufacturing, the singulation, dicing, scribing, cleaving, cutting, and facet treatment of wafers or glass panels is a critical processing step that typically relies on diamond cutting, with speeds of 30 cm/sec for flat panel display as an example. After diamond cutting, a mechanical roller applies stress to propagate cracks that cleave the sample. This process creates poor quality edges, microcracks, wide kerf width, and substantial debris that are major disadvantages in the lifetime, quality, and reliability of the product, while also incurring additional cleaning and polishing steps. The cost of de-ionized water to run the diamond scribers are more than the cost of ownership of the scriber and the technique is not environmentally friendly since water gets contaminated and needs refining that itself adds the costs. By advance techniques dyes on the wafers are getting smaller and closer to each other that limit the diamond scribing. 30 μm is a good scribing width and 15 μm is challenging. Since diamond scribing uses mechanical force to scribe the substrate, thin samples are very difficult to scribe. The FPD industry is seeking to reduce glass thicknesses to 150-300 μm from conventional 400-700 μm that is used currently and scribing the plates is the major issue. Indeed the FPD industry is looking to use thin tempered glass instead of ordinary glass for durability.
Laser ablative machining is an active development area for singulation, dicing, scribing, cleaving, cutting, and facet treatment, but has disadvantages, particularly in transparent materials, such as slow processing speed, generation of cracks, contamination by ablation debris, and moderated sized kerf width. Further, thermal transport during the laser interaction can lead to large regions of collateral thermal damage (i.e. heat affected zone). Laser ablation processes can be dramatically improved by selecting lasers with wavelengths that are strongly absorbed by the medium (for example, deep UV excimer lasers or far-infrared CO2 laser). However, the above disadvantages cannot be eliminated due to the aggressive interactions inherent in this physical ablation process.
Alternatively, laser ablation can also be improved at the surface of transparent media by reducing the duration of the laser pulse. This is especially advantageous for lasers that are transparent inside the processing medium. When focused onto or inside transparent materials, the high laser intensity induces nonlinear absorption effects to provide a dynamic opacity that can be controlled to accurately deposit appropriate laser energy into a small volume of the material as defined by the focal volume. The short duration of the pulse offers several further advantages over longer duration laser pulses such as eliminating plasma reflections and reducing collateral damage through the small component of thermal diffusion and other heat transport effects during the much shorter time scale of such laser pulses. Femtosecond and picosecond laser ablation therefore offer significant benefits in machining of both opaque and transparent materials. However, machining of transparent materials with pulses even as short as tens to hundreds of femtosecond is also associated with the formation of rough surfaces and microcracks in the vicinity of laser-formed hole or trench that is especially problematic for brittle materials like glasses and optical crystals. Further, ablation debris will contaminate the nearby sample and surrounding surfaces.
A kerf-free method of cutting or scribing glass and related materials relies on a combination of laser heating and cooling, for example, with a CO2 laser and a water jet. [U.S. Pat. No. 5,609,284 (Kondratenko); U.S. Pat. No. 6,787,732 UV laser (Xuan)] Under appropriate conditions of heating and cooling in close proximity, high tensile stresses are generated that induces cracks deep into the material, that can be propagated in flexible curvilinear paths by simply scanning the laser-cooling sources across the surface. In this way, thermal-stress induced scribing provides a clean splitting of the material without the disadvantages of a mechanical scribe or diamond saw, and with no component of laser ablation to generate debris. However, the method relies on stress-induced crack formation to direct the scribe and requires [WO/2001/032571 LASER DRIVEN GLASS CUT-INITIATION] a mechanical or laser means to initiate the crack formation. Short duration laser pulses generally offer the benefit of being able to propagate efficiently inside transparent materials, and locally induce modification inside the bulk by nonlinear absorption processes at the focal position of a lens. However, the propagation of ultrafast laser pulses (>˜5 MW peak power) in transparent optical media is complicated by the strong reshaping of the spatial and temporal profile of the laser pulse through a combined action of linear and nonlinear effects such as group-velocity dispersion (GVD), linear diffraction, self-phase modulation (SPM), self-focusing, multiphoton/tunnel ionization (MPI/TI) of electrons from the valence band to the conduction band, plasma defocusing, and self-steepening [S L Chin et al. Canadian Journal of Physics, 83, 863-905 (2005)]. These effects play out to varying degrees that depend on the laser parameters, material nonlinear properties, and the focusing condition into the material.
Kamata et al. [SPIE Proceedings 6881-46, High-speed scribing of flat-panel display glasses by use of a 100-kHz, 10-W femtosecond laser, M. Kamata, T. Imahoko, N. Inoue, T. Sumiyoshi, H. Sekita, Cyber Laser Inc. (Japan); M. Obara, Keio Univ. (Japan)] describe a high speed scribing technique for flat panel display (FPD) glasses. A 100-kHz Ti:sapphire chirped-pulse-amplified laser of frequency-doubled 780 nm, 300 fs, 100 μJ output was focused into the vicinity of the rear surface of a glass substrate to exceed the glass damage threshold, and generate voids by optical breakdown of the material. The voids reach the back surface due to the high repetition rate of the laser. The connected voids produce internal stresses and damage as well as surface ablation that facilitate dicing by mechanical stress or thermal shock in a direction along the laser scribe line. While this method potentially offers fast scribe speeds of 300 mm/s, there exists a finite kerf width, surface damage, facet roughness, and ablation debris as the internally formed voids reach the surface.